1. Field of the Invention
The present invention relates to cavity resonators and, more particularly, to an apparatus and method for providing temperature compensation in TE.sub.101 mode and TM.sub.010 mode cavity resonators.
2. Description of the Prior Art
The use of a cavity resonator for high frequency filtering purposes is well known in the art. The cavity resonator is generally realized in the form of an enclosed housing that is constructed from a material having a high conductivity. This conductive housing furnishes large areas for current to flow and confines electromagnetic fields therein. Such a housing exhibits a natural resonant frequency and generally has a very high quality factor (Q). However, when the cavity resonator housing is subject to temperature variations, there are corresponding variations in the natural resonant frequency and the Q due to thermal expansions and contractions of the housing material. For example, if a cavity resonator housing is constructed of copper, which has a coefficient of expansion of about 9.3 ppm/.degree.F., an increase in temperature will cause a corresponding increase in the housing dimensions and thereby a decrease in the resonant frequency. Specifically, in the above case the frequency will decrease by 9.3 Hz/MHz/.degree.F., which is too large a variation for applications requiring a high operating selectivity. It is therefore desirable to provide compensation for such thermal variations so as to maintain consistent cavity resonator frequency characteristics.
One prior art method for providing temperature compensation for the thermal expansion and contraction of a cavity resonator housing has been to construct the housing from a material commonly known as Invar. Invar is a metallic compound having a coefficient of expansion of approximately 0.5 ppm/.degree.F. Thus, when a cavity resonator constructed of Invar is subject to temperature variations, the resulting frequency variations are very small when compared to a cavity resonator constructed of copper. However, the resulting frequency variation with temperature of a cavity resonator constructed with Invar may still be too large in high selectivity applications. Furthermore, due to a high cost of Invar it would be more desirable to construct a cavity resonator housing of a more conventional and less costly material, such as copper, copper plated steel, or copper plated aluminum.
Another prior art method for providing temperature compensation in a cavity resonator housing is described in U.S. Pat. No. 4,423,398, entitled, Internal Bi-Metallic Temperature Compensating Device For Tuned Cavities, issued Dec. 27, 1983. This patent described how a strip of temperature sensitive bi-metallic material is used to provide temperature compensation by way of a reformation of the bi-metallic material over temperature. A problem with this method, however, is that the temperature compensating effects can be somewhat inconsistent because of a dependence on a large number of variables; i.e., position of the strip, dimensions of the strip, relative angle of the strip, material of the strip, etc. It is therefore desirable to overcome the above-mentioned shortcomings while providing a simple, low cost, highly reliable and accurate temperature compensation scheme for high frequency cavity resonators.